Particularly in the case of office devices in which a coated tape is drawn from a dispenser reel and after giving up its coating (e.g. an adhesive film, cover-up film, etc.) must be wound in uncoated form back onto the take-up reel in the device, due to the not only constantly varying, but also reciprocally different speeds of the two reels there is a need to provide between same a driving gear and to compensate for the speed differences which occur by the use of a suitable slip clutch between one of the reels and the associated driving or gear wheel. Due to the constricted space available in office devices use is generally made of slip clutches coaxial to the corresponding reel core.
In a conventional form for such slip clutches, the driving or gear wheel has a centrally projecting spindle portion which projects through a central opening of a central king pin fitted to the associated reel in such a way that it projects axially out of said opening at its end. To said projecting end is fixed a metal clip whose two side legs engage from the outside over the reel king pin and engage under pretension resiliently against its outer face. As a result of this metal clip connected rigidly in the rotation direction to the driving or gear wheel a connection by frictional resistance is created between its resilient legs and the reel king pin, which acts as a slip clutch, if there is a relative speed difference between the reel and the driving or gear wheel. Such a solution is described in U.S. Pat. No. 4,718,971. However, these slip clutches are relatively complicated to assemble, which particularly applies in the case of automatic assembly and this constitutes their main disadvantage. In addition, they are expensive from the material standpoint, because it is generally necessary to use metal clips as spring elements. To ensure that when using such an office device in which a film, such as an adhesive film, has to be transferred from a carrier foil to a substrate, at the end of application the film coating is removed from the tape (i.e. separated from the film coating still left on the tape) without undesirable further movement taking place to the reels, this would mean in the case of such slip clutches that the pretension of the metal clip would have to be so strong that continuous tape unwinding would be very difficult. This disadvantage is obviated in the known devices in that a number of locking/unlocking means separate from the slip clutch are provided and are either operated by the user by pressing on a button on the casing and then disengaging the reels or, as in U.S. Pat. No. 4,718,971, by the springing out of the application foot releasing the locking action in use counter to the effect of a back-tension spring. Metal springs have a relatively large dispersion in manufacture, so that the correct operation of such slip clutches presupposes that from the outset a relatively large pretensioning of the metal springs must be ensured, so that an adequate pretension is always obtained even for minimum values out of the tolerance range. If an undesired lubrication occurs between the legs of the metal clip and the smooth engagement surface of the associated king pin, there is a marked, sudden reduction of friction, which can in turn lead to an undesired easy action of the force closure. The known slip clutch is unable to compensate angular variations between the pivot pins of the reel and the gear wheel.
Similar disadvantages occur in another known solution, where use is made of a "spring belt" gear between the two reels, whose spring belts slips in the case of an unequal speed of the reels. Although here minor angular variations can be compensated, the disadvantages resulting from frictional resistance grip persist and assembly is complicated.
DE-OS 36 38 722 discloses a coaxially acting slip clutch in which a clutch cover whose internal diameter is provided with all-round grooves or serrations is in locking engagement with the ends of thin elastic angle catches projecting radially outwards from the clutch wheel. The reel with the core is mounted on this clutch cover, so that drivers constructed on the latter engage in suitable slots inside on the reel core. The disadvantages linked with a slip clutch operating purely by frictional resistance are admittedly avoided in this known slip clutch, but it has a complicated assembly, which more particularly applies for the fitting of the elastic, radial catches, especially in the case of automatic assembly. In addition, the known slip clutch is expensive from the material standpoint due to the large number of components used and is therefore relatively expensive to use in general terms. However, it is not able to compensate even a minor angular displacement between the pins of the reel and the driving wheel.